A new technique for producing coatings on a wide variety of substrate surfaces by kinetic spray, or cold gas dynamic spray, was recently reported in two articles by T. H. Van Steenkiste et al. The first was entitled “Kinetic Spray Coatings,” published in Surface and Coatings Technology, vol. 111, pages 62-71, Jan. 10, 1999 and the second was entitled “Aluminum coatings via kinetic spray with relatively large powder particles”, published in Surface and Coatings Technology 154, pp. 237-252, 2002. The articles discuss producing continuous layer coatings having high adhesion, low oxide content and low thermal stress. The articles describe coatings being produced by entraining metal powders in an accelerated gas stream, through a converging-diverging de Laval type nozzle and projecting them against a target substrate. The particles are accelerated in the high velocity gas stream by the drag effect. The gas used can be any of a variety of gases including air or helium. It was found that the particles that formed the coating did not melt or thermally soften prior to impingement onto the substrate. It is theorized that the particles adhere to the substrate when their kinetic energy is converted to a sufficient level of thermal and mechanical deformation. Thus, it is believed that the particle velocity must exceed a critical velocity high enough to exceed the yield stress of the particle to permit it to adhere when it strikes the substrate. It was found that the deposition efficiency of a given particle mixture was increased as the inlet air temperature was increased. Increasing the inlet air temperature decreases its density and thus increases its velocity. The velocity varies approximately as the square root of the inlet air temperature. The actual mechanism of bonding of the particles to the substrate surface is not fully known at this time. The critical velocity is dependent on the material of the particle. Once an initial layer of particles has been formed on a substrate subsequent particles bind not only to the voids between previous particles bound to the substrate but also engage in particle to particle bonds. The bonding process is not due to melting of the particles in the particles because the temperature of the particles is always below their melting temperature.
One aspect of the technique is that the particles are entrained in the converging side of the nozzle, pass through a narrow throat and then are expelled from the diverging section of the nozzle onto a substrate. One difficulty that can arise is that with certain particles sizes the throat can rapidly become plugged. In a recent related United States application, filed Apr. 5, 2002 and assigned Ser. No. 10/117,385 this was addressed through a modification of the kinetic spray technique that involves injection of the particles into the diverging region of the nozzle and then entraining them in the accelerated gas stream. The technique removes clogging of the nozzle throat as a limitation and reduces the wear on the nozzle.
Using the basic technique attempts were made to coat electrical contact substrates with tin particles. The particles adhered to and coated the electrically conductive substrates. During impact fracturing occurs in the particles as they plastically deform and adhere to a substrate and other particles. It was found, however, upon subsequent bending of the coated substrates to form them into the required terminal shape the particles broke internally along these fracture lines and left a fragment of the original tin particle at the break on the substrate. These broken particles negatively affect the substrate surface. The present invention is directed to a method of overcoming the particle fracturing behaviour and to design a coating that could withstand severe bending without damage.